1. Field of the Invention
The present invention generally relates to noise abatement and, more particularly, to a device for reducing noise caused by a fluid passing near a body.
2. Brief Description of the Prior Art
Fixed wing aircraft typically include engines, a fuselage, airfoils, and a tail section. Each airfoil is typically a solid section having a leading edge, a trailing edge, a first end, and a second end. A retractable flap, having a first flap end and a second flap end, is usually positioned adjacent the trailing edge of the airfoil, between the first end of the airfoil and the second end of the airfoil. When the flap is deployed, the flap provides additional lift to the airfoil.
Significant advancements have been made in making aircraft engines significantly quieter, primarily through the use of high-bypass-ratio engine technology. However, airframe noise remains a problem. Airframe noise is defined as sound from an aircraft induced solely by airflow not related to the engine during operation, which can rival that of the engine during aircraft landing. This noise poses a threat to the successful certification of subsonic aircraft.
As shown in FIG. 1, recent research using detailed experimental measurements and computations of an airflow field at first and second end portions A, B of a flap C indicates that vortices D are generated as a result of the lift of the flap C. These vortices are fed by a cylindrical shear layer E over most of the flap chord F, which is the streamwise length of the flap C. Numerical simulations indicate that this cylindrical shear layer E is highly unstable, resulting in strong flow field fluctuations adjacent the flap end portions A, B. These fluctuations have further been shown to create significant noise G in frequency ranges consistent with acoustic measurements of noise from the flap end regions. For full-scale subsonic transport aircraft, range is about 400 to 2500 Hz, while for model-scale measurements the typical frequency range is 5 to 30 kHz.
Currently, no known devices are being used to reduce the airframe noise generated at the end portions of the flap. A publication by A. S. Hersh et al. entitled xe2x80x9cInvestigation of Acoustic Effects of Leading-End Serrations on Airfoilsxe2x80x9d, J. Aircraft, Vol. 11, No. 4, April 1974 (hereinafter referred to as xe2x80x9cthe Hersh publicationxe2x80x9d) has shown that leading end serrations can reduce narrow-band tones radiated from stationary and rotating airfoils at low Reynolds numbers, but this is an entirely different application associated with a different noise-generation mechanism, i.e., the noise generation mechanisms in the Hersh publication are not caused by the cylindrical shear layer instability associated with the formation of vortices adjacent the flap end portions. In the Hersh publication, the serrations probably eliminate a leading edge separation bubble. Although the suspected noise-generation mechanism was discussed in subsequent publications, no method for eliminating the mechanism was proposed.
To help reduce the noise levels created by a body moving through a fluid, the present invention generally includes a method of abating noise caused by a shear layer created by fluid flow adjacent an end of the body. The primary step is thickening the shear layer adjacent to the end of the body. The shear layer is thickened by positioning protuberances adjacent the end of body or, alternatively, by slots formed by the end of the body. The protuberances thicken the shear layer by creating horseshoe-shaped vortices; by thickening the shear layer, the instability mechanism responsible for generating fluctuations and thus noise is weakened.
It is therefore an object of the present invention to reduce the noise created by a body passing through a fluid or, alternatively, the noise caused by a fluid traveling over, through, or around a body.
These and other advantages of the present invention will be clarified in the Brief Description of the Preferred Embodiments taken together with the attached drawings in which like reference numerals represent like elements throughout.